(1) Field of the Invention
The present invention relates to a ceramic trimmer capacitor. More particularly, the present invention relates to a ceramic trimmer capacitor in which the reliability against cracking of the ceramic board used as a dielectric member and the dependency of the electrostatic capacitance on the rotor load are improved.
(2) Description of the Prior Art
A typical example of a conventional trimmer capacitor is first described with reference to FIGS. 1 through 3 of the accompanying drawings.
Referring to FIG. 1, a disc-shaped rotor member 1 is formed by die casting of zinc or the like, and on the bottom of the rotor member 1, there is formed a substantially semicircular rotor electrode 1a, and a semi-annular convex portion 1b having the same height as that of the rotor electrode 1a is formed and projected on the bottom of the rotor member 1 so that a good balance is maintained between the rotor electrode 1a and the convex portion 1b on the bottom of the rotor member 1 (see FIG. 2). A thin ceramic board 3 acting as a dielectric member has a substantially oval shape (see FIG. 2) and a substantially semi-circular stator electrode 7 is formed on one surface of the ceramic board 3 by printing with a silver paste or the like. A frame 4 formed from an insulating synthetic resin has a substantially cup-like shape and a hot terminal 5 formed of a thin metal plate is attached to the bottom 4f of the frame 4 by insertion molding, press fitting or the like. One end of the hot terminal 5 is bent in a fine groove 4g formed (see FIGS. 1 and 3) on the inner bottom face 4a of the frame 4 to form a resilient contact portion 5a. As shown in FIG. 3, a linear portion 4c and an arcuate portion 4h are formed on the inner circumferential face of the peripheral wall 4b of the frame 4.
The ceramic board 3 is placed on the inner bottom face 4a of the frame 4 so that the stator electrode 7 is directed downward and the linear side portion 3a of the ceramic board 3 is fitted in the linear portion 4c of the frame 4 to hold, the ceramic board 3 immobile. The contact portion 5a of the hot terminal 5 resiliently engages the stator electrode 7 to provide electric conduction between the hot terminal 5 and the stator electrode 7. The rotor member 1 is placed on the ceramic board 3 so that the rotor electrode 1a and the semi-annular convex portion 1b are directed downward.
A metal shaft 2 has a head 2a thereof abutting against the bottom of a driver groove 1c of the rotor member 1, and the other end 2b of the metal shaft 2 is inserted through the rotor member 1, ceramic board 3 and frame 4 and projected outside from the bottom 4f of the frame 4 and, the other end 2b of the metal shaft 2 is caulked to the bottom 4f of the frame 4 through a grounding terminal 6. The rotor member 1 is rotatably supported on the frame 4 by means of the shaft 2 and simultaneously, the ceramic board 3 is pressed and gripped between the rotor member 1 to the inner bottom face 4a of the frame 4 by means of the shaft 2. Furthermore, the rotor member 1 is electrically connected to the grounding terminal 6 through the shaft 2.
When the rotor member 1 is rotated by an adjustment tool such as a screwdriver, the confronting area of the rotor electrode 1a of the rotor member 1 and the stator electrode 7 is changed to appropriately adjust the capacitance of the capacitor.
In the conventional trimmer capacitor having the above-mentioned structure, in order to reduce the manufacturing cost, an inexpensive synthetic resin material inevitably is used for the frame 4, and the formation of sink marks or warping is often caused on the inner bottom face 4a of the frame 4 because of molding shrinkage, resulting in reduction of the eveness of the inner bottom face 4a. Furthermore, since the stator electrode 7 is formed on the ceramic board 3 by printing or the like, the ceramic board 3 has a difference in thickness corresponding to the thickness of the stator electrode 7 and therefore, a gap that cannot practically be neglected is inevitably present between the ceramic board 3 and the inner bottom face 4a of the frame 4. Accordingly, at the step of caulking the shaft 2 or at the time of applying a pressing load onto the rotor member 1 for adjustment of the capacitance, the ceramic board 3 is irregularly distorted and the degree of the closeness between the ceramic board 3 and the rotor member 1 is degraded, resulting in variation of the capacitance. Namely, the dependency characteristic of the electrostatic capacitance on the rotor load is degraded, which is one of the defects of the conventional ceramic trimmer capacitor. Furthermore, the distortion is increased with increase of the rotor load, and since a bending force is imposed on the ceramic board 3, it sometimes happens that the ceramic board 3 breaks. This is another fatal defect of the conventional ceramic trimmer capacitor.
As a means for eliminating the foregoing defects, a structure in which an annular convex portion is formed integrally on the outer periphery of the inner bottom face 4a of the frame 4 so that only the periphery of the ceramic board 3 is supported by this annular convex portion (see Japanese Utility Model Application No. 161199/80) has been proposed. In this ceramic trimmer capacitor, good improvement in the stability against cracking of the ceramic board and the dependency of the static capacitance on the pressing load can be attained. However, in this proposed ceramic trimmer capacitor, since the annular convex portion has an annular shape covering 360.degree., some sink marks or warps are formed on the surface of the annular convex portion by molding shrinkage of the frame 4. Accordingly, if the thickness of the ceramic board 3 is reduced, the reliability against cracking of the ceramic board under a pressing load is insufficient to some extent and it is impossible to sufficiently improve the dependency characteristic of the electrostatic capacitance on the pressing load.